1. Field of the Invention
The invention relates to breathable composites and, more particularly, breathable housewrap composites made of a breathable filled polyolefin film laminated to an open-mesh fabric and particularly to such composites having improved tensile, tear strength and stiffness characteristics.
2. Related Art
It is known to use porous polyolefin films in housewrapping applications. Housewrap films are attached or secured outside of the sheathing in the walls of homes, especially in cold climates. Housewrap materials must be permeable to water vapor to allow water vapor to escape from the wall to which the film is secured. Otherwise, water trapped in the wall may cause a condition known as "sweating" or rotting which may ultimately damage the wall. On the other hand, the film must be sufficiently impermeable to air to insulate the wall against wind and water. Further, the film must have adequate tensile and physical properties such as break strength, elongation, tear strength, shrinkage and puncture strength to avoid damage during installation and to facilitate durability.
One commercially available film heretofore used as a housewrap is made of high density polyethylene flash spun into fibers and pressed to form the film. The resulting flash-spun HDPE film has excellent breathability, but suffers from both a high air permeability and a relatively low tear strength. Thus, such housewrap is subject to damage during shipment and installation. Another commercially available film employed as a housewrap is melt blown, spun-bonded polyethylene. Like the flash-spun HDPE fiber film, the spun-bonded polyethlyene has excellent breathability, but likewise has a high permeability to air and even worse tensile properties, i.e. break strength, tear strength and puncture resistance. Thus, there is an unfilled need for a housewrap film with both breathability and good physical and tensile properties.
It is known to prepare porous polyolefin films by stretching a precursor film filled with calcium carbonate. "Breathable" films which are gas/vapor permeable and liquid impermeable have been described in U.S. Pat. No. 4,472,328, assigned to Mitsubishi Chemical Industries, Ltd. The Mitsubishi patent describes a breathable polyolefin film prepared from a polyolefin/filler composition having from 20 percent to 80 percent by weight of a filler such as a surface treated calcium carbonate. A liquid or waxy hydrocarbon polymer elastomer such as a hydroxy-terminated liquid polybutadiene was found to produce a precursor film that could be monoaxially or biaxially stretched to make a film breathable. The breathable film described by Mitsubishi is also described in Great Britain Patent No. 2,115,702, assigned to Kao Corporation. The Kao patent further describes a disposable diaper prepared with a breathable film as disclosed by the Mitsubishi patent. The breathable film is used as a backing for the diaper to contain liquid.
U.S. Pat. No. 4,350,655, assigned to Biax Fiber Film, describes a porous polyolefin film containing at least 50 percent by weight of a coated inorganic filler. The precursor film is formed without the addition of an elastomer by employing an inorganic filler surface coated with a fatty acid ester of only silicon or titanium. The precursor film is then rolled between horizontally grooved rollers. Cold stretching of the precursor film at a temperature below 70.degree. C. produces a porous film. Some of the resulting films were stated to be both vapor and liquid permeable, however, and at least one film (Example 3) was stated to be permeable to air.
Nonwoven cross-laminated fibrillated film fabrics of high density polyethylene are also known such as, for example, as described in U.S. Pat. No. 4,681,781 assigned to C-I-L, Inc., and are commercially available, for example, under the trade designation CLAF from Conwed Plastics. The CLAF films, for example, are spread during the manufacturing process therefor and then laminated at the crossing points to form a thin, open mesh fabric of exceptional strength and durability. The CLAF films are known to be laminated to materials such as paper, film, foil, foam and other materials by lamination and extrusion coating techniques, or by sewing or heat sealing, adding significantly to the strength of the reinforced material without adding substantial bulk, according to the manufacturers. Reportedly successful uses of CLAF fabrics include shipping sacks for cement, fertilizer and resins, shopping, beach and tote bags, consumer and industrial packaging such as envelopes, form, fill and seal pouches, and tape backing, disposable clothing and sheeting, construction film and wraps, insulation backing, and reinforcement for reflective sheeting, tarpaulins, tent floors and geotextiles, and agricultural ground covers, insulation and shade cloth.